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Opened up the top end and found the top piston ring in bits, so I guess this is the problem. Looks like piston will require replacement as the groove between the rings has bent and worn in one spot.
Valves appear OK, allbeit a bit black. Cylinder smooth with no damage.

Resistance measurements for the Stator and Pulse generator found and checked against mine, which would appear to be OK.
Due to earlier comments regarding low compression (and its about all thats left), I guess its time to go into the engine.
Would valves or piston be a good place to start checks? Can the head and barrel be removed with the engine in the frame? Any advise or tips would be appreciated.
For information, Stator checks are:
Pulser Coil (red-green) - 100ohms +-20%
Exciter (black/red - red/white) - 12.7ohms +-20%
Charge coil - (ground - yellow) - 0.65ohms +-20%
- (white - yellow) - 0.16ohms +-20%
Thanks again for all the help and advice.

2003 450 EXC, 40Hrs (700 miles) from new.
This is a pickup from an earlier thread, as the bike still fails to run despite mine and local dealers best efforts (I got the bike back after paying several hours labour and dealer no further forward). The story so far…….
·Bike died on a ride out and failed to re-start.
·Coil, Plug Lead, Cap and Plug all checked (lead, plug cap and plug replaced). Good fat blue spark when earthed to head and either kicked or e-start.
·Carb stripped, cleaned and operation checked.
Bike still failing to start so taken to dealer for assistance. Dealer carried out the following:
·Set valve clearances (were slightly tight).
·Removed e-starter and cleaned gear.
·Replaced CDI with one from stock – same result.
·Swapped Carb with one from another 450 EXC – same result.
The current status is that the bike still fails to start by either kick or e-starter. If boost charger used (to spin a bit faster), with tickover up, the engine will eventually start but sounds really flat and dies soon after starting. Will keep running as long as revved.
Exhaust removed (in case blocked) – same result.
I suspect a piston ring, but when running there is no excessive blue smoke? Dealer checked compression when spinning fast (to bypass decompression) and he only gets 70psi. Sounds a bid low to me, but don’t really know what to expect for this engine with auto decompression.
Dealer seems to think something in the engine has tightened up, but I am not convinced as I can kick fairly easy. When the bike does run, it feels flat like a fuelling problem, but after all that has been done it must be a problem in the engine.
Sorry if this is a bit ‘war & peace’, but before I embark on the engine strip, I would really appreciate thoughts or assistance.

Just removed the Ignition Cover to check and found everything covered in oil, which I would guess is my problem?
All I need to do now is work out where the oil has come from and how to fix it....
Any suggestions welcomed

I have a 2003 EXC450 which whilst out riding, died suddenly and now fails to start.
Spark Plug and Cap checked and I appear to be getting a good spark (kill switches OK). Carb cleaned and checked. Occasional 'pop' when attempting to Kick or E-Start.
Any advise or assistance would be appreciated..